The welding together of facing edge portions of metal strips or sheets using an induction coil supplied with high frequency current, e.g. at 10 kHz and up to 450 kHz, for taking advantage of skin effect, to induce current in the strips or sheets which flows in opposite directions at the faces of the edge portions, for taking advantage of "proximity effect", is well known in the art. See, for example, U.S. Pat. Nos. 2,763,756; 3,037,105; 4,197,441 and 4,845,326. The practice has been to use a single induction coil in various relations to the parts to be welded together. For example, in the welding of strip folded around an axis to form a tube, the induction coil has encircled the axis either outside or inside the tube. Since the induced current must flow in a closed path, the current flowing along the edge faces which are being brought together is useful, but since the current must also flow along the inner or outer peripheral surfaces of the tube to complete the path, heating, which is not useful, also occurs at such surfaces.
In an attempt to reduce such non-useful losses, induction coils which are located at only one side of the surfaces of the metal part or parts, sometimes known as a "pancake" or "split induction" coils have been developed. However, with such coils, it is difficult to obtain the desired coupling between the parts and the coils and in addition, the impedance of such coils is relatively high which causes current load matching problems and difficulty in providing sufficient power to a coil for producing rapid heating of the edge faces.
The latter problems have assumed more importance because vacuum tube power sources, which can feed loads of high impedance, are being replaced by solid state power sources which require a relatively low impedance load.
There are difficulties and additional expense in adding a second induction coil to a system using a single induction coil. Aside from the additional cost of adding a second induction coil, there is the problem of providing connections to the second coil, which must permit the parts to be fed between two coils and when one coil is to be within a tube, there are support problems, current supply and size change problems. Even if those skilled in the art may have considered the addition of a second induction coil, which is not admitted, they would have believed that each coil would be required to supply part of the power required when a single coil is involved and that the problems encountered when a second coil is used would not be worth the effort.
Surprisingly, I have found that even though there are the mechanical problems, such as support, current supply and size change, the electrical efficiency or economy in energy use, when two coils are used, are so great that the mechanical problems can be tolerated. Furthermore, the use of two induction coils can substantially reduce the load impedance so that the two induction coils can be fed by power sources requiring low impedance loads, such as a solid state power source.
Thus, I have found that the efficiency of induction welding can be substantially increased with a pair of induction coils disposed as described hereinafter.
Tests which have been conducted have provided the following results:
______________________________________ System Power Efficiency ______________________________________ Contact current supply 22 Kw 100% Outside coil around tube 48 Kw 46% as in U.S. Pat. No. 3,037,105 Inside coil around tube axis 50 Kw 44% Both foregoing coils 33 Kw 66% parallel fed Split induction coil outside 44.5 Kw 49% tube as in FIG. 32 of U.S. Pat. No. 4,197,441 Split induction coil inside 36 Kw 61% tube under "V" Both split induction coils 29 Kw 76% parallel fed ______________________________________
The tests were conducted in connection with a stationary twelve inch diameter steel pipe having a wall thickness of one-quarter inch and a power source supplying electrical energy at a frequency of about 200 kHz. The edge faces of the folded metal strip were brought together at a weld point and were spaced apart for six inches in advance of the weld point to provide a "V" in advance of the weld point. The contact current supply system, which was similar to the system shown in U.S. Pat. No. 2,818,489, was assumed, for comparison purposes, to have an efficiency of 100% since electrical energy absorbed other than at the edge faces to be welded is minimal and the amount of power required to raise the temperature at the weld point to 600.degree. F. in 1.0 seconds was measured. Impeders, members of magnetic material, e.g. ferrite, were placed under the "V" during each of the tests.
From the foregoing table, it will be apparent that the use of induction coils is not as efficient electrically as a contact system, but of course, induction coils, even though they have electrical disadvantages, do have other advantages for some purposes.
From the comparison of the data for a single coil coaxial with the tube axis, either inside or outside the tube, it would be expected that if the power were equally supplied by two coaxial induction coils, the power requirement would be 49 Kw (98/2 Kw). However, unexpectedly, to obtain the same results with two coaxial induction coils, rather than one, only 33 Kw of power were required providing an electrical efficiency improvement of 22%.
Similarly, with a single split induction or "pancake" coil, it would be expected that if the power were equally supplied by the two coils, the power requirement would be 40.25 Kw (80.5/2 ). Again, unexpectedly with two such coils only 29 Kw of power were required providing an electrical efficiency improvement of 27% with respect to a single split induction coil outside the tube and an efficiency improvement of 15% with respect to a single split induction coil inside the tube.